This invention relates to the reduction of CO and SO.sub.x components in the flue gas discharged from regenerators associated with catalytic cracking units. More particularly, this invention relates to injecting particles of bastnaesite into fluidized catalytic cracking (FCC) units to reduce emissions from the catalyst regenerator.
In the petroleum industry, high boiling hydrocarbon feedstocks are charged to FCC units so that, by contact with a moving bed of catalyst particles, the feedstocks are converted to a more valuable hydrocarbon product, such as gasoline, having a lower average molecular weight and a lower average boiling point than the feedstock from which it was derived. The most typical hydrocarbon feedstock treated in FCC units consists of heavy gas oil, but on occasion such feedstocks as light gas oils, naphthas, reduced crudes, and even whole crudes are subjected to catalytic cracking to yield low boiling hydrocarbon products.
Catalytic cracking in FCC units is usually accomplished by a cyclic process involving separate zones for catalytic reaction, steam stripping, and catalyst regeneration. The hydrocarbon feedstock is blended with an appropriate amount of catalyst particles, and the mixture so produced is then passed through a catalytic reactor, commonly called a riser, wherein a catalytic cracking reaction zone is maintained such that at a temperature between about 800.degree. and 1100.degree. F. the feedstock is converted into gaseous, lower boiling hydrocarbons. After these lower boiling hydrocarbons are separated from the catalyst in a suitable separator, such as a cyclone separator, the catalyst, now deactivated with coke deposited upon its surfaces, is passed to a stripper. In the stripper, the deactivated catalyst is contacted with steam so as to convert some of the coke to hydrocarbon product vapors, which are then combined with the vapors received from the cyclone separator, and the mixed vapors are then transferred to other facilities for further treatment. Meanwhile, catalyst particles are recovered from the stripper, and because only a small proportion of the coke is removed in the stripper, the catalyst is introduced into a regenerator wherein, by combustion in the presence of an oxygen-containing gas such as air, the remaining, larger proportion of coke is removed and the catalyst reactivated. The cyclic process is then completed by blending the reactivated catalyst particles with the feedstock entering the riser of the FCC unit.
One recognized problem in the conventional FCC process resides in the incomplete combustion in the regenerator. Due to the relatively low temperature of combustion in the regenerator, usually between about 900.degree. and about 1300.degree. F., the flue gas contains carbon dioxide and carbon monoxide in a ratio of 0.8:1 to about 10:1, CO.sub.2 :CO, with the carbon monoxide concentration often being as high as 15 mole percent. Such high concentrations of carbon monoxide are a source of concern inasmuch as carbon monoxide is a pollutant, and this concern has recently resulted in numerous methods to reduce the amount of carbon monoxide discharged from FCC regenerators and the like. In general, these methods aim to reduce the polluting effects of carbon monoxide by incorporating into the cracking catalyst particles metal components that are active for catalyzing the reaction of CO with oxygen at the relatively low temperatures encountered in FCC regenerators. Alternatively, the metal CO oxidation promoter may be incorporated on particles having substantially no catalytic cracking activity that circulate in the FCC unit with the catalyst particles. The metal promoters useful for such purposes include the Group VIII metals and rhenium, as disclosed in U.S. Pat. No. 4,072,600, chromium, as disclosed in U.S. Pat. No. 2,647,860, Group IB, IIB, VIB, VIIB, and VIII metals, as disclosed in U.S. Pat. No. 3,364,136, and certain rare earth metals, such as cerium, as taught in Netherlands Pat. No. 73/00884 (equivalent to U.S. Pat. No. 3,823,092), and lanthanum, as taught in U.S. Pat. No. 4,137,151.
The processes described in the foregoing prior art references usually entail substantial costs due to the expense involved in impregnating selected metals onto catalyst particles. For example, if a certain rare earth element is desired as the CO oxidation promoter, substantial costs are incurred in extracting the selected rare earth element from an ore and separating it from the other metals also extracted from the ore. In addition, added costs are involved in modifying cracking catalysts so as to include the selected rare earth element thereon. Usually, the selected metal is incorporated upon the catalyst particles by an impregnation-calcination procedure which substantially increases the cost in manufacturing a cracking catalyst. Thus, the prior art approach to reducing CO emissions from FCC units is costly and one aim of the art has been to reduce such costs and at the same time obtain significant reductions in CO emissions.
In addition to the problem of reducing CO emissions from FCC units, another major pollution problem is presented when the hydrocarbon feedstock contains organic sulfur compounds. Ideally, the sulfur compounds in a feedstock treated in an FCC unit are converted to H.sub.2 S in the catalytic reaction and stripping zones so that all the contained sulfur in the feedstock is recovered as H.sub.2 S with the product vapors and later separated therefrom by contact with an aqueous alkanolamine solution. But in practice, it has been found that some sulfur components remain (or are converted to forms which remain) with the coke on the deactivated catalyst recovered from the stripper. Hence, when the coke is combusted in the regenerator, a flue gas containing SO.sub.x compounds is produced.
The flue gas, if untreated, is a source of pollution. Although about 90-95% of the sulfur compounds entering an FCC unit with the feedstock are ultimately removed as H.sub.2 S and other gaseous sulfur compounds, the remaining 5-10% left with the coke and converted to SO.sub.x compounds in the regenerator represents a significant environmental and engineering problem. For a typical FCC unit handling a feedstock containing about 1.5 weight percent sulfur components (calculated as elemental sulfur) fed at a rate of about 50,000 barrels per day, the amount of SO.sub.x compounds discharged from the regenerator in one day is between about 3.0 and 10 tons (calculated as SO.sub.2).
Because of the concern created by the discharge to the atmosphere of SO.sub.x compounds in such large quantities, various methods have been devised to reduce SO.sub.x emissions from FCC units to environmentally tolerable levels. Recently, attempts have been made to reduce such SO.sub.x emissions by recycling with the catalyst particles in the FCC unit a metal-containing component, commonly called a "sulfur getter," that reacts in the regenerator with the gaseous SO.sub.x compounds to yield a solid sulfur compound. The produced sulfur compound is then reconverted to the active "sulfur gettering" form by passage through the riser and stripper wherein the solid sulfur compounds are decomposed to release H.sub.2 S. The released H.sub.2 S is then recovered with the low-boiling hydrocarbons produced in the stripper and riser and then separated from said low-boiling hydrocarbons, as stated before, by contact with an alkanolamine solution.
One method illustrating the use of a "sulfur getter" is described in U.S. Pat. No. 3,835,031, wherein magnesium oxide is incorporated on the catalyst for the purpose of scavenging SO.sub.x compounds. In the regenerator, the magnesium oxide reacts with the SO.sub.x compounds to produce magnesium sulfate, thereby preventing the release of SO.sub.x compounds from the regenerator. As the catalyst particles are recycled through the catalytic cracking and steam stripping zones maintained in the riser and stripper, respectively, the magnesium sulfate is converted back to magnesium oxide while the contained sulfur is released as hydrogen sulfide and collected with the low boiling hydrocarbon products. Thus, the catalyst particles, when recycled to the regenerator again, contain a magnesium compound (i.e., magnesium oxide) in an active form for removing SO.sub.x.
Similar processes have been taught in U.S. Pat. Nos. 3,699,037, 4,071,436, 4,137,151, 4,146,787, and 4,153,535. These references disclose many metals for reducing the amount of SO.sub.2 discharged from FCC regenerators. Usually, the metals are incorporated onto the catalyst itself, but some references also disclose FCC processes wherein particles separate from the catalyst are introduced into the FCC unit and recycled with the catalyst to control sulfur oxides emissions. U.S. Pat. No. 4,071,436, for example, teaches a process wherein particles of reactive alumina are circulated with the catalyst particles. And U.S. Pat. No. 4,146,463 discloses thirteen specific metals plus the Group II and rare earth metals for removing SO.sub.2 produced in FCC regeneators, and the metals so disclosed are taught as useful when impregnated on the catalyst itself, incorporated on an inert substrate (i.e., inert for cracking hydrocarbons), or utilized as a powdered oxide.
One difficulty residing in the foregoing processes is that the "sulfur getter" material must be prepared by processes involving substantial manufacturing costs. An object of the present invention, therefore, is to improve upon the prior art processes by utilizing an abundant and inexpensive material, namely bastnaesite, as the "sulfur getter," thereby substantially reducing the cost of removing SO.sub.2 from FCC regenerators. It is yet another object of the invention to introduce bastnaesite into the catalytic cracking cycle for the purpose of simultaneously lowering SO.sub.x and CO emissions from the regenerators of FCC units. It is yet another object of the invention to reduce SO.sub.x and CO emissions from FCC regenerators and the like by introducing bastnaesite, either in a particulate form separate from the catalyst or in a form physically attached to the catalyst, into the FCC cycle for the purpose of scavenging SO.sub.x and/or catalyzing the conversion of CO to CO.sub.2 in the FCC regenerator. These and other objects will become more apparent in view of the following description of the invention.